I've come across Big History as promoted by David Christian & co., and as someone with a multidisciplinary background it strongly appeals to me, but it seems like the kind of thing where you can slip into Jared Diamond-type hackery quite easily.
How reputable is the field (?) among professional historians, and if it's taken seriously, what materials should I start with as an undergraduate (i.e. someone looking for something more substantive than the MOOC directed at highschoolers)?
Are there any institutional concentrations of expertise? Macquarie and Amsterdam seem to be the main two names but even Amsterdam only has two relevant faculty as far as I can tell. Do any other institutions have major concentrations of "macrohistory" scholarship, at least, if not specfically under the Big History umbrella?
It depends on who is writing it and what it is about. I think you will like this 'Monday Methods' thread about the Annales School led by /u/commiespaceinvader, who also answered Why do so many academic historians look down on military history?
/u/agentdcf has previously compared Jared Diamond to other "big historians"
/u/eternalkerri has previously devised a taxonomy of history publishing markets as part of a thread asking a similar question to yours.